The invention relates to an hydraulic transmission with at least two pumps supplying pressure medium to at least one hydraulic motor, with at least one of the at least two pumps furnishing fluid to the hydraulic motor only intermittently, when the fluid-furnishing demand reaches a value which cannot be met by the other pump or pumps.
In a known hydraulic transmission for driving the wheels of a vehicle, there are provided two constant-output pumps, i.e., two pumps which when their mechanical inputs are driven at constant speed furnish fluid at a constant non-adjustable volumetric flow rate. One pump furnishes fluid to an hydraulic motor which drives the vehicle wheels; the other furnishes fluid to another consumer. If this other consumer is not to be operated, then the second pump can be connected in circuit with the hydraulic motor which drives the vehicle wheels, so that such hydraulic motor is driven by the two pumps jointly, when a higher vehicle speed is desired.
Such an arrangement has the disadvantage that when the second pump is brought into circuit with the hydraulic motor there occurs a sudden jolt-like acceleration of the vehicle. In addition, the use of constant-volumetric-flow-rate pumps does not make it possible to drive the hydraulic motor which drives the vehicle wheels in an optimal manner.
It is known to provide an hydraulic transmission with an adjustable-volumetric-flow-rate pump, i.e., a pump which when its mechanical input is driven at constant speed can nevertheless be adjusted to furnish fluid at different volumetric flow rates. In particular, it is known to provide a hydraulic transmission with a single pump, and this an adjustable-volumetric-flow-rate pump, to overcome some of the disadvantages just mentioned. However, the cost of an adjustable-volumetric-flow-rate pump large enough to constitute the sole pump for an hydraulic transmission in a vehicle is great, making the cost of the transmission very high.